Recent studies show that stroke survivors often exhibit abnormal muscle synergy activation patterns. Restoring impaired muscle synergies may help rehabilitate impaired upper limb function. The objective of this study was to develop an upper limb muscle synergy training interface and test the feasibility of training new muscle synergies on healthy participants. A game was interfaced with a robot that can guide isokinetic movement while measuring endpoint force applied on a handle in six directions. A previously developed model estimated the endpoint force vector of a desired muscle synergy activation and was displayed on a monitor to provide an intuitive visual cue for training. Two healthy participants were trained to activate three muscle synergy patterns when reaching in each direction. Results confirmed the feasibility of training unfamiliar muscle synergy activations for healthy participants and provided insights for applying the training interface to the rehabilitation of impaired muscle synergies for stroke survivors